This invention relates to the field of interferometry, and particularly to scanning interferometers intended for use in spectrometry. More specifically, its primary focus is on improving Michelson interferometers intended for use in infrared Fourier transform spectroscopy.
My previously filed application, Ser. No. 790,457, filed Apr. 25, 1977, and also titled "Refractively Scanned Interferometer", discloses an interferometer wherein scanning is accomplished by means of a single, uncompensated refractive element, preferably wedge-shaped in cross-section, used in conjunction with stationary reflectors in both interferometer arms. The significant advantages of such an arrangement, which are discussed in detail in that application, include a very substantial improvement in motion control during scanning, which for the first time makes it practical to use Fourier transform spectroscopy as an on-line technique for such purposes as stack monitoring, medical gas analysis, liquid and gaseous process control, and the analysis of gas chromatography fractions.
The use of a wedge-shaped prism for interferometer scanning can introduce a problem, unless the orientation and direction of motion of the prism are properly designed. This problem is the translatory motion, or lateral displacement, of the optical beam during scanning motion of the prism. The problem is particularly significant when retro-reflectors are used as the mirrors for the interferometer arms.
The primary concern of this application is to prevent lateral optical beam displacement due to scanning motion of the wedge. There is a second form of beam displacement, which depends on wavelength rather than wedge position. Specifically, the angle through which a beam of radiation is bent on passing through the wedge will vary with wavelength, due to the chromatic dispersion of the wedge index of refraction. In a "compensated" wedge design described to me by Aaron Kassel, a consultant, in January, 1976, he proposed a double-wedge arrangement, in which a stationary compensating wedge was included, having the same apex angle as the moving wedge in order to avoid the chromatic dispersion effect. Such a double-wedge arrangement is subject to the same problem of lateral beam displacement due to wedge motion as is my single wedge design. The concepts covered by this application are required with either the single, uncompensated scanning wedge design, or the double-wedge design in which an additional stationary wedge "compensates" for the chromatic dispersion effect.